DEV: Use method instead of constant for discobot badge names

This change refactors the code a bit so that a plugin could easily
replace which badge is awarded when completing the discobot new user
tutorial and advanced tutorial.

By adding a static method and putting the BADGE_NAME constant inside of
that method we can simply call that method now instead of the constant.
A plugin could then `class_eval` that method and replace it with
whatever badge name they choose. This is way cleaner than having the
plugin change the frozen constant! eeek.

plugins/discourse-narrative-bot/spec/discourse_narrative_bot/advanced_user_narrative_spec.rb

@@ -725,7 +725,7 @@ RSpec.describe DiscourseNarrativeBot::AdvancedUserNarrative do
                                                "topic_id" => topic.id,
                                                "track" => described_class.to_s)
 
-        expect(user.badges.where(name: DiscourseNarrativeBot::AdvancedUserNarrative::BADGE_NAME).exists?)
+        expect(user.badges.where(name: DiscourseNarrativeBot::AdvancedUserNarrative.badge_name).exists?)
           .to eq(true)
       end
     end

plugins/discourse-narrative-bot/spec/discourse_narrative_bot/new_user_narrative_spec.rb

@@ -1007,7 +1007,7 @@ describe DiscourseNarrativeBot::NewUserNarrative do
           )
 
           expect(user.badges.where(
-            name: DiscourseNarrativeBot::NewUserNarrative::BADGE_NAME).exists?
+            name: DiscourseNarrativeBot::NewUserNarrative.badge_name).exists?
           ).to eq(true)
         end
       end

plugins/discourse-narrative-bot/spec/discourse_narrative_bot/track_selector_spec.rb

@@ -319,7 +319,7 @@ describe DiscourseNarrativeBot::TrackSelector do
                 )
 
                 BadgeGranter.grant(
-                  Badge.find_by(name: DiscourseNarrativeBot::NewUserNarrative::BADGE_NAME),
+                  Badge.find_by(name: DiscourseNarrativeBot::NewUserNarrative.badge_name),
                   user
                 )